1. Field of the Invention
The present invention relates to a device comprising a nitride semiconductor (InxAlYGa1−X−YN, 0≦X, 0≦Y, X+Y≦1) and particularly to an electrode formed on a p-type nitride semiconductor layer.
2. Discussion of the Related Art
Nitride semiconductors have been used for light emitting devices such as light emitting diode (LED) and laser diode (LD), light receiving devices such as solar batteries and optical sensors, and electronic devices such as transistors and power devices. Especially, light emitting diodes utilizing a nitride semiconductor have been widely used in a traffic signal, a large-scale display, and a back light source etc. The light emitting diodes utilizing a nitride semiconductor are also suitably used in an image scanner, in a media such as DVD as a light source for an optical disc etc. which stores large information, for a light source in a communication system, in a printing equipment, in a light source for a lighting, and so forth. The nitride semiconductor described above is specifically comprised of AlxInyGa1−x−yN (0≦x, 0≦y, x+y<1).
This nitride semiconductor light emitting diode basically has a structure where an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer are laminated in sequence on an insulating substrate such as sapphire. Because an electrode cannot be formed on the rear face of the substrate, an n-electrode is disposed on the n-type nitride semiconductor layer which is exposed by a partial etching from the p-side nitride semiconductor layer side, and the p-electrode is formed on the remaining p-type nitride semiconductor layer which was not subjected to the etching. That is, the p-electrode and the n-electrode are disposed on the same face where the semiconductor layer is laminated. The device having such a structure emits light with the wavelength mainly shorter than 550 nm.
Although the resistance of the p-type nitride semiconductor layer is lowered by annealing or electron beam irradiation etc., the resistance is still higher than the resistance of the n-type nitride semiconductor layer, and the electric current doesn't spread easily in the p-type nitride semiconductor layer. Therefore, the p-electrode is formed on the entire face of the p-type nitride semiconductor layer. By this construction, the electric current is spread out the entire surface of the p-type semiconductor layer, and a uniform distribution of emission is obtained. When the side where the semiconductor layer is laminated is made to an observation side, the emission is blocked by the p-electrode. By this reason, a transparent electrode is used for the p-electrode and the emission is extracted through the p-electrode.
Here, a film electrode that contains nickel (Ni) and gold (Au) has an excellent ohmic contact with the p-type nitride semiconductor layer and has a transparent property, so that it is desirably used as a p-electrode, as described in Japanese Unexamined Patent Publication No. 2000-299528.
However, an electrode that contains gold (Au) absorbs light having shorter wavelength than 550 nm. Consequently, there has been a problem when the electrode containing gold is used for the p-electrode, a major portion of light is absorbed by the p-electrode and a sufficient amount of light can not be extracted.
Rhodium (Rh) can be cited as a material capable of solving such a problem. Rhodium is known to have a preferable reflectivity in the region from ultraviolet to infrared. Moreover, Rhodium has ideal physical properties as the electrode for a light emitting device and a light receiving device, such as low electrical resistivity, and excellent heat resistance and corrosion resistance. However, because rhodium has a large contact resistance against a nitride semiconductor, there was a need to improve the contact resistance so as to be used in a light emitting device, as described in Japanese Unexamined Patent Publication No. H11-274562.
The present invention has been made in order to solve the above problems and an object of this invention is to provide a nitride semiconductor device having a good external quantum efficiency, by reducing the light absorption in the electrode formed on the p-type nitride semiconductor layer.